Electrical watthour meters are commonly used to measure the amount of electrical power consumed by an electrical utility customer. Watthour meters have the well known gauges to indicate the amount of power consumed. Typically, a watthour meter has a socket-type housing that plugs into a base or service panel having contacts connected to the building wiring. The socket-type meter being separate from the base or panel enables the meter to be easily removed and replaced without removing the entire meter base or interfering with the building wiring.
The socket-type meters commonly plug into the electrical meter base or service panel with blade-like stab connectors on the meter inserted into spring loaded jaws of cooperating receptacle-like contacts in the meter base or panel. The connectors between the socket-type meter and the meter base or panel must be designed so that the meter can be operated efficiently and safely at a certain maximum current or load level. The connections must provide a certain level of current carrying capacity, and poor contact between the connectors impedes current flow and produces corresponding heat rise at the connectors. Safety is also of paramount importance with electrical devices carrying large electrical loads, and minimal heat rise caused by the plug-in contacts when the line is operated at high loads is the objective to ensure safety. Accordingly, standard industry certification testing requires that a socket-type watthour meter inserted into a standard meter base or panel and operated at a predetermined load level must not exceed a predetermined maximum heat rise. A principal design requirement in avoiding excessive heat rise is to provide positive electrical contact between the blade-like connectors on the meter and the spring-loaded jaws of the electrical receptacles in the meter base or panel. That is, if the spring-loaded jaws of the receptacles produce good, reliable surface contact with the blade-like connectors on the meter, then the connections may not produce a heat rise problem.
In recent years, meter socket adapters or "extenders" have been installed between the watthour meter and the meter base or panel. These meter socket adapters have been used for various reasons, including providing surge protection, providing a means for converting a bottom-connected watthour meter installation to a socket-type watthour meter installation, and providing for a change in the angle of the meter face, to name a few. Meter socket adapters commonly include a socket-type housing having electrical connectors formed by bus bars which provide blade-like contacts on a back side of the housing for engaging the terminals in the meter base or panel. The other side of the contacts in the adapter housing comprise spring-loaded jaws at the ends of the bus bars that removably receive the plug-in blade-like connectors on the watthour meter.
Meter socket adapters create an additional plug-in-type blade and receptacle connection interposed in the line between the meter and the meter base, when compared with a socket-type meter that is not used with a socket adapter. Because of the additional electrical connections in the line brought about by use of the adapter, additional total heat rise is inherently produced. In addition, the socket adapters are contained in a closed housing, which entraps heat and, therefore, adds to the heat rise. Since heat rise for meter bases or panels without adapters is limited to a certain maximum amount by industry certification standards, it becomes a challenge for a line containing a meter adapter to also meet these heat rise certification standards. Certification standards (presently known as UL 414) for a conventional socket-type meter base or panel (without use of an adapter between the meter and base) require a maximum heat rise limit of 65.degree. C. above ambient. The heat rise test for the meter base contacts is rigorous, and the heat rise test becomes increasingly more difficult to pass as the load (current) increases. The meter base or panel is subjected to a sequence of tests which include passing the rated current through the meter base or panel until constant temperatures are attained at the contacts, removing and reinserting a simulated meter for a large number of cycles while the contacts are in the heated condition, cooling to room temperature and removing and reinserting the simulated meter for a large number of cycles, cycling at 120% rated current for several hours followed by cooling, and passing 100% rated continuous current through the meter until constant temperatures are attained. The test involves measuring temperatures at potential hot spots in the contacts continuously throughout the sequence of tests.
Meter socket adapters that have previously been used in the electrical utility industry have not been required to pass these UL certification tests which are applicable to socket-type watthour meter bases or panels. An objective of the present invention is to provide a meter socket adapter that can be interposed in the line between the meter and the meter base or panel and still have the entire assembly pass the standard certification testing for socket-type watthour meter bases or panels. This includes the objective of passing the 65.degree. C. (above ambient) maximum total heat rise test for meter bases or panels carrying high current loads. Many meter bases or panels are rated at high maximum loads of 175 amps continuous or more, and an objective of this invention is to provide an adapter that can pass the heat rise test for these rated current levels.
Meter socket adapters using presently available jaw-type receptacles are unsuccessful in limiting total heat rise sufficiently to pass the UL 414 certification test. In fact, most would fail miserably.
As mentioned previously, a principal requirement in avoiding excessive heat rise is to provide a positive electrical contact between the blade-like connectors of the meter and the spring loaded jaws of the electrical receptacles that receive the meter. The previous jaw-type contacts have included a variety of mechanisms for increasing surface contact or pressure contact, or both, between the blade and jaws. The removal and reinsertion steps in the heat rise test sequence in most cases quickly causes the spring loaded jaws to lose tension because of poor spring design that does not maintain a sufficient level of spring "memory" for a large number of insertions. Spring tension loss can result in poor contact that increases heat rise beyond acceptable certification test limits. Thus, a prior art connector that passes the UL 414 heat rise test for meter sockets may not pass the same test when the connector is used in an adapter tested for heat rise under the same standards.
In addition to heat rise, certification tests also require that the contacts meet "insertion force" specifications. Meter insertion force (the force required to insert the meter blade connectors into the jaw-type receptacle connectors of the adapter) must remain within certain limits for a number of repetitive removal and reinsertion cycles. Thus, although it may be desirable to have a strong spring force in order to improve surface contact for passing the heat rise test, excessive spring force can cause the adapter to fail the insertion force test. (Unreasonably high insertion force is undesirable because it indicates extreme difficulty in the ability to plug the meter into the socket.) On the other hand, if insertion force is too low, particularly after a few removal and reinsertion cycles of the test, it indicates poor electrical contact between the blade and jaws of the receptacle which leads to excessive heat rise.
In previously known plug-in type blade and receptacle designs, many different spring mechanisms for improving the contact force of the jaws are used. However, they often lose their spring force after only a few cycles of the insertion force test. This is particularly true for receptacle jaws biased by heavy, stiff spring wire, which deforms excessively after only a few insertions of the blades and therefore does not retain a level of insertion force to pass the heat rise test.
The present invention provides a meter socket adapter having connectors that are capable of meeting total heat rise and insertion force test standards commonly used in the certification of standard socket-type electrical meters. The invention, in fact, has passed UL 414 certification standards for heat rise (65.degree. C. maximum heat rise above ambient at 185 amps continuous maximum load) and the related insertion force tests.